In commercial vehicle trailers, there is a need for a display system separate from the motor truck for certain data, such as the mileage and the condition of built-in electronics of the trailer. This is particularly important for the mileage of the trailer, since the trailer often travels behind different tractors, and, as a result, the mileages of the tractor and trailer generally do not agree. By recording the mileage of the trailer in such a way, it is possible, for example, to ascertain the maintenance intervals thereof more accurately.
Trailer systems of the general type under consideration are known. For example, WO 99/44017 describes an odometer for a trailer in which one wheel is equipped with its own wheel-speed sensor. For this purpose, a speed sensor of an anti-lock braking system (ABS) can also be used as the sensor. The signal of the sensor is fed via a cable to its own odometer evaluating electronics having a microcontroller and a display. The electronics contain a battery and a power-saving circuit.
Also, a display system for various data of a trailer is described in a brochure for trailer vehicles titled “Infomaster für Anhängefahrzeuge 446 303 . . . ” WABCO PROSPEKT, [Online] Aug. 1998 (1998-08), pages 1-2, XP002446211. In this known system for a trailer equipped with an ABS, the speed signal of an ABS wheel-speed sensor of the trailer is passed to both the ABS electronics and a separate display unit. In most cases this display unit is mounted on the side of the rear part of the trailer such that it is visible from the outside. The distance signal for such a display unit is usually derived from the closest ABS wheel sensor. For this purpose, what is known as a Y cable is used, which branches to the ABS electronics and to the display unit. However, it is a complicated matter to lay such a cable, which is also costly to manufacture.